1. Field of the Invention
This invention relates to a primer composition for a plastics substrate as well as a method for treating the surface of the plastics substrate by using such primer. More particularly, this invention relates to a primer composition for a plastics substrate comprising one or more specific diacryloyloxy compounds dissolved in an organic solvent and to a method for treating the surface of a plastics substrate comprising coating the surface of the plastics substrate with the primer, coating a curable resin-containing top-coating finish over the primer-coated plastics substrate and curing the finish to thereby prepare a desired coated product.
2. Description of the Prior Art
In recent years, shaped transparent plastics articles such as shaped articles of polycarbonate, polymethyl methacrylate polyvinyl chloride, polystyrene or cellulose butyrate and so forth have been used to replace glass in many applications because of their transparency and light weight. But the fact is that, to date, their use has been limited because of their vulnerability to abrasion and loss of transparency.
Several methods have been proposed to eliminate such defects of the shaped transparent plastics article by coating its surface with a transparent finish to provide a protective film coating. And furthermore, many attempts have heretofore been made to provide the surface of the plastics, except for transparent plastic, with a protective film coating to improve the performance of their surface such as chemical resistance, wear resistance and surface hardness.
Usually, a curable resin-containing finish is used as a protective finish for protecting the surface of the shaped plastics article and various kinds of such finishes have been proposed. Above all, finishes containing a carbon functional silane compound such as vinyl group containing silane compound, epoxy group containing silane compound, amino group containing silane compound and so forth are particularly interesting in practical use, since the film coating obtained therefrom has excellent properties.
Some plastics, however, have poor affinity for a cured resin coating covering the surface of the plastic substrate and with these only a very weak bond is obtained between the surface of the plastic substrate and the coating. It is, therefore, necessary that such plastic substrates be treated with a primer to form a primer coating before they are given the final coating.
In consequence of various studies in search for a method of protecting the surface of plastic substrates, especially transparent plastic substrates, there has been successfully prepared a series of silane compounds-containing thermosetting resin compositions that are capable of providing the surface of a transparent plastic substrate with a cured resin coating which satisfies the requirements of high wear resistance, surface hardness, heat resistance and chemical resistance (Japanese Patent Application No. 123,078/1977) and proposed a method for pre-treating a plastic substrate, which has poor affinity for silane compounds-containing a curable protective finish, such as polycarbonate and polymethylmethacrylate, by using diepoxy compounds (Japanese Patent Application No. 67,916/1978).
The above-mentioned pretreatment using diepoxy compounds brings about a remarkable improvement of bond strength between a plastic substrate and curable protective finish which contains a silane compound containing an epoxy group. However, when certain kinds of protective finishes containing a silane compound are used, it has been found that the bond strength between the plastic substrate and the protective coating is lowered after the boiling water treatment.
Therefore, a fully satisfactory pretreatment method for improving significantly the bond strength between the cured resin coating and the plastic substrate has not been obtained in the prior art.